1. Field of the Invention
The present invention relates to the field of back-illuminated screens.
2. Prior Art
The advent of digital high definition (HD) video technology is causing a phenomenal demand for HD televisions (HDTV) and HD display devices with large screen sizes having high brightness characteristics. Several display technologies are poised to address this demand; including Plasma Display Panel (PDP), Liquid Crystal Display (LCD), and Rear Projection Display (RPD) devices that use micro-display imagers such as a digital micro-mirror device (DMD) or a liquid crystal on silicon (LCOS) device. The cost and brightness performance of the latter display technology is highly dependent on the efficiency of the screen system it uses. The designers of such display systems are constantly in search of more cost effective, efficient screen systems that would offer high levels of uniformity, contrast and brightness.
The function of a rear projection screen is to accept an image projected on one side (herein after referred to as the projection side of the screen) and to display this image to viewers on the opposite side (herein after referred to as the viewing side of the screen). The screen must interact with the projected image; hence the physical and optical properties of the screen are responsible for the screen, and subsequently the entire projection system, viewing characteristics. The physical and optical properties of the rear projection screen, ultimately translate into a set of parameters that govern its performance, including brightness gain, brightness uniformity, transmission efficiency, resolution and diffused reflectance. The angular brightness of a rear projection screen is best described in terms of its brightness gain, which is the ratio of measured brightness of a screen to the brightness of an ideal Lambertian screen as a function of the viewing angle. By Lambertian, we mean that the distribution of the light transmitted by the screen would have the same brightness or luminance when viewed from any viewing angle. The brightness uniformity of a rear projection screen describes the spatial brightness uniformity across the screen and is obtained by expressing the fractional change of brightness compared with the average brightness within a specified range of viewing angles. The transmission efficiency of a rear projection screen generally expresses the screen efficiency in terms of the fraction of light that passes through the screen and more specifically in terms of the fraction of the incident light that is scattered by the screen within some specific viewing angle. The resolution of a rear projection screen is one of the most important performance parameters, as it limits the fineness of details that can be usefully projected. The resolution properties of a rear projection screen are best expressed by the modulation transfer function, which governs the contrast transfer characteristics of the screen as a function of spatial frequency. The diffuse reflectance of a rear projection screen determines its performance sensitivity to ambient light in terms of the amount of ambient light that is diffusely returned to the viewing area from the screen. The relationship between the aforementioned performance parameters of a rear projection screen and their theoretical models are described in detail in Ref [29].
In addition to the projection screen characteristics, the performance of a rear-projection display system is governed by other factors such as: (1) the type of projection, e.g., from micro-display based digital projectors, or from laser beam scanners, etc.; (2) the projection and viewing geometries, e.g., from a single or an array of projectors, the projection image maximum incident angle on the screen, the size of the screen and the size and shape of the viewing area; (3) the brightness and uniformity of the projected image; (4) the resolution and contrast of the projected image; (5) the level of ambient light at the projection side and viewing side of the screen; and (6) the viewer perception. Of particular interest to the scope of this invention are screen systems that can effectively be used in conjunction with rear projection systems that utilize an array of multiple projectors to generate the projected image such as those described in Ref [1-5] and [18]. The performance of this type of rear projection display system is strongly affected by the variations in the angle of incidence of the light rays generated from the array of multiple projectors, which would cause: (1) viewing angle dependent variations in the brightness viewed across the screen; and (2) the blending regions to have brightness that varies with the viewing angle, which would make the image blending regions become visible at some viewing angles. This is because the brightness of an image that is diffused from a rear projection screen varies as a function of both the angle of incidence that the image makes with respect to the projection screen, and the angle at which the viewer views the image on the projection screen, Ref [29]. As a result images seamlessly tiled, calibrated and blended at one viewing angle position will have visible seams when viewed from another slightly different viewing position. This type of viewing angle brightness sensitivity in tiled rear projection display systems cannot be overcome solely by the edge blending and calibration techniques described in prior art Ref [19]-[27].
In its most basic form, a rear projection display screen would be transmissive and may include a light scattering element, or diffuser. Numerous variations of light scattering elements have been developed, including volume scatterers, surface scatterers, holographic diffusers, beads, lenticular elements and the like. While a diffuser can serve the basic function of a projection screen, additional features are often required in selected applications. For example, structures that suppress the reflection and transmittance of ambient light are often incorporated into projection screens. Controlled scattering angles have also been used to maximize the luminance (brightness) of the viewable light within a range of viewing angles. Uniformity enhancing mechanisms such as Fresnel lenses have also been placed behind or incorporated into the back of rear projection screens.
An illustration of a prior art rear projection system is shown in FIG. 1A and FIG. 1B. As shown, a projector 10 projects an image on the projection side of a screen assembly 12 which is comprised of a Fresnel lens collimation screen 13 and a projection screen 14. The image from the projector 10 is focused in the proximity of the projection screen 14. Before the light reaches projection screen 14, the light rays are redirected (collimated) by Fresnel lens collimation screen 13 to impinge on the projection screen 14 at substantially a normal angle of incidence. As shown in FIG. 1B, the projection screen 14 may be a dual lenticular structure having a rear and front lenticular surface 15 and 16, respectively. The rear lenticular surface 15 approximately focuses the light onto the front lenticular surface 16, in the region between black stripes 18 thus allowing the projected light 19 focused by the lenticular structure 15 to exit the projection screen 14 towards the viewer. Black stripes 18 absorb a substantial portion of the incident ambient light, thereby increasing the contrast of the screen. To complete the screen and control the effective scattering profile, diffusion stripes 17 are incorporated into or onto the screen regions between the black stripes 18. In the prior art example illustrated in FIG. 1B, the additional diffusion stripes determine the degree of scattering in the vertical axis along the direction of the lenticular surfaces 15 and 16. The two lenticular surfaces 15 and 16 function to provide a controlled amount of scatter only in the direction normal to the lenticular axes.
Prior art Ref. [6]-[12] disclose variations on this basic rear projection screen system approach. These schemes tend to work well for rear projection systems comprised of a single projector, however they are not effective in tiled multi projectors rear projection systems. The difficulty associated with these prior art screen technologies is their inability to overcome the viewing angle sensitivity associated with tiled rear projection display systems as explained earlier. Furthermore, many prior art screens cannot readily support the projection overlap in tiled displays that is typically used to blend the images along the seams of adjacent projectors. For example, in a prior art Ref. [2] Fresnel field lens approach, little or no overlap would be allowed because each projector must typically have a distinct Fresnel lens. The Fresnel lens simply cannot compensate for light emanating from different spaced locations. Because little or no overlap is allowed, the projected image from each projector must typically be precisely matched in size and location with the corresponding Fresnel lens to minimize the visible seams. This greatly impacts the alignment tolerance and stability of the resulting screen system. Further, it may be difficult to mask slight variations in luminance or color coming from adjacent projectors.
Toward overcoming the aforementioned viewing angle sensitivity associated with tiled rear projection display systems, prior art Ref [1] describes a rear projection pre-screen comprised of an optical faceplate made of a fibrous crystal that emulates a wave guiding effect which would collimate the light from the multiple projectors prior to being diffused, thus helping in reducing the projection system viewing angle brightness sensitivity. Pursuant to the same objective, prior art Ref [2] describes an approach in which one or more lenses are added adjacent to each projector in order to reduce the angle of incidence that the image makes with respect to the projection screen. Ref [3]-[5] aims at achieving the same objective by using a pre-screen layer that is comprised of a plurality of micro-lenses designed to partially collimate the light projected from the multiple projectors, thus reducing the angle of incidence that the projected images make with respect to the projection screen. However, the approaches described in Ref [1], [2] and [3]-[5] are only effective when each of the tiled projector's field-of-view is relatively small (less than 20°), which causes the projection depth to be large. The techniques described in Ref [1], [2] and [3]-[5] may be adequate for large venue tiled projector display systems in which the projection depth is not a parameter of critical importance. However, in the rear projection array display system described in Ref [18], the projection depth is a parameter of paramount importance and limiting the tiled projector's field-of-view to less than 20° will cause such a display system to have a large depth. In order to reduce the depth of the type of rear projection array display systems described in Ref [18] to the range of depth of other flat panel display systems, the tiled projector's field-of-view should be substantially larger than 20°. As a result the techniques described in prior art Ref [1], [2] and [3]-[5] for overcoming viewing angle brightness sensitivity in tiled rear projection display systems cannot be effectively used in conjunction with the rear projection array display system such as that described in Ref [18].
An objective of this invention is, therefore, to demonstrate a rear projection screen system that can effectively address the viewing angle brightness sensitivity associated with tiled rear projection display systems in particular those designed to achieve small projection depth. Achieving such an objective would have a substantial commercial value, as it would enable low form-factor and compact packaging of tiled rear-projection display systems.